This invention relates to the area of active vibration control. Specifically, the invention relates to improvements in active mounts for fixed wing applications. More specifically, this invention is directed to a system for cancelling two tones which are relatively close in frequency, as in the case of the primary (and/or secondary) disturbance frequencies of pairs of turbofan or turboprop engines.
In the realm of active noise and vibration control, there are three implementation approaches: active noise control, which uses an inverse-phase sound wave to cancel the disturbance signal; active structural control, which vibrates a structural component at a frequency to cancel the input disturbance (noise and/or vibration); and active isolation control, where an actuator in a mount is reciprocated at the proper frequency, phase and amplitude to cancel the input disturbance (which, again, may be a structural vibration or in the audible range, in which case it is experienced as noise). The decoupling feature of the present invention can be utilized with each of these three implementation approaches.
Active mounts for controlling vibrational input from an engine to the support structure are known. For example, commonly assigned U.S. Pat. No. 5,174,522 issued to Hodgson teaches the use of an active fluid mount for vibration cancellation. Systems which actively control vibration or sound by using an out of phase cancellation signal are also known and include U.S. Pat. Nos. 4,677,676 to Eriksson, 4,153,815 to Chaplin, 4,122,303 to Chaplin et al., 4,232,381 to Rennick et al., 4,083,433 to Geohegan, Jr. et al., 4,878,188 to Zeigler, Jr., 4,562,589 to Warnaka et al., 4,473,906 to Warnaka et al., 5,170,433 to Elliott, 4,689,821 to Salikudden et al., and 5,133,527 to Chen et al. These systems utilize digital microprocessors (processors) to control or minimize mechanical vibration or ambient noise levels at a defined location or locations, as for example noise or vibration experienced in an aircraft cabin or within an automobile passenger compartment. Generally, these systems are responsive to at least one external input signal such as a synchronizing tachometer signal and/or error signal as supplied by various types of sensors such as microphones, accelerometers, etc. These systems strive to reduce to zero, or at least minimize, the recurring sound and/or vibration.
Multiple-input, multiple-output (MIMO) systems are required to adequately compensate for the vibrations of plural turbofan or turboprop engines. In active control systems of the above-mentioned type, it is generally required to have an input signal for each tone to be canceled which is supplied to an adaptive filter and/or a processor which is indicative of the frequency content and/or amplitude/phase of the input source, i.e., indicative of the disturbance signal. Particularly, it is usually required to have two or more analog or digital waveforms, such as a sine and cosine wave, that are synchronized with (at the same frequency as) the input source signal for providing the appropriate information to the processor and/or adaptive filter. These waveforms will be utilized in computing the appropriate frequency and amplitude of a cancellation signal in accordance with a particular algorithm such as least mean square (LMS) and filtered-x LMS algorithms.
Many such algorithms have difficulty processing two tones which are close in frequency such as in the case of a right engine operating at a first frequency N.sub.1R and a left engine operating at a second frequency N.sub.1L which is the same or nearly the same as the first frequency. Turbofan and turboprop engines typically have four tones that are objectionable: N.sub.1R which corresponds to the frequency of the right fan or prop, N'.sub.1L which corresponds to the frequency of the right turbine, N.sub.1L which corresponds to the left engine fan or prop and N'.sub.1L which corresponds to the left engine turbine frequency. These similar tones (N.sub.1R and N.sub.1L or N'.sub.1R and N'.sub.1L) can cyclicly reinforce one another creating a particularly objectionable beat frequency. The relative closeness of the two tones can cause the system to become unstable as the algorithm seeks to find an optimal cancellation solution.
In practice, each of the error sensors of such a system will pick up all four of the engine disturbance frequencies, to some degree. The most general controller objective is for each actuator to provide a cancellation force at each of the four tones. The controller would provide a signal segment of sufficient amplitude and phase inverted to cancel each of the individual four components, then superpose the four signal segments into a single cancellation signal (complex sine wave) to be fed to the actuator. When any two of the four tones are relatively close in frequency (and generally there are two pairs of such tones), the control algorithm can have difficulty converging to a stable set of actuator signals.
The present invention provides decoupling of the response to the two tones having similar/identical frequencies by proper positioning of the sensors and the actuators. Preferably, both the sensors and actuators can be placed in the primary disturbance path between the power plant (or engine) and the support structure. (In the case of active isolation control, the actuator will necessarily be in the primary disturbance path. In the case of active noise cancellation, the microphones will not be in the primary disturbance path.) In addition, the error sensors must be widely spaced enough to prevent cross-coupling of the closely spaced frequencies (N.sub.1R and N.sub.1L, for example). By spatially separating the error sensors, the magnitude of the signal N.sub.1L detected by the sensors positioned to monitor the N.sub.1R signal and visa versa, will be small enough that it can be ignored (i.e., will be at least an order of magnitude smaller) or may be filtered out by the signal conditioner.
In another aspect of the invention, pairs of sets of force transmission elements within the active mount are positioned such that each element can transmit a vertical force component and a horizontal force component. Further, one of the force transmission elements from each of the mounts is targeted to focalize its cancellation force (i.e., the elastic center, the point at which the axes of force intersect, is at or beyond the center of gravity of the power plant). Focalization is well known in the mounting art, and is more particularly described in U.S. Pat. Nos. 2,175,999 issued to Taylor and 2,241,408 issued to Lord which are hereby incorporated by reference. Preferably, the two force transmission elements are orthogonally oriented. Further, in one embodiment, each transmission element is preferably oriented at a 45.degree. angle to the horizontal. In a second alternative embodiment, the orthogonal actuators may be arranged to act along horizontal and vertical axes, respectively. The actuators may be tuned absorbers, electromagnetic, electrohydraulic or piezoelectric.
Various other features, advantages and characteristics of the present invention will become apparent after a reading of the following specification.